Incidence of false-positive results for assays used to detect antibiotics in milk

The incidence of false-positive results from milk assays fur antimicrobial agents was determined for composite milk samples collected from 407 lactating dairy cows with a history of no antibiotic treatment for a minimum of 30 days. Milk samples were also cultured for bacteria and analyzed for somatic cell count. Mean herd prevalence of intramammary infections (+/-SEM) caused by Streptococcus agalactiae and Straphylococcus aureus was 3.3 +/- 2.8 and 20.2 +/- 9.5% of lactating cows, respectively. All 407 milk samples were assayed for antibiotics with three commercial tests; a fourth test was used to assay 391 samples. Samples were assayed twice with each test, and if the results from these repetitions did not agree, a third assay was performed and the result obtained in two of the three repetitions was reported. Because samples were only collected from cows will no antimicrobial treatment for a minimum of 30 days, positive assays were considered to be false-positive results. Three test kits did not yield any false-positive results, one test kit had 5 false-positive results of 407 samples collected (specificity, 98.8%). Although there was a trend for false-positive samples to have a higher somatic cell count than negative samples, the low incidence of false-positive results did nor allow a meaningful comparison. We conclude that the incidence of false-positive results is very low when testing milk from cows that have a history of no clinical mastitis or antimicrobial treatment.